The present invention relates generally to lasers, and more particularly, to frequency doubling, solid-state lasers including a lasant material and a nonlinear optical material.
Frequency doubling lasers have been used for some time to produce green light lasers (see references 1 and 2 listed at the end of the description). Currently these lasers typically use a pumping light source, such as a laser diode, to cause a lasant material, such as neodymium yttrium vanadate (Nd:YVO.sub.4), located within a laser resonator to lase thereby generating a fundamental wave at a wavelength at and/or about 1064 nm (see reference 3). The laser resonator also includes an input mirror, an output mirror, and a nonlinear material, such as potassium titanate phosphate (KTP). The nonlinear material receives the fundamental wave generated by the lasant material as its input and produces second harmonic waves of green light at wavelengths at and/or about 532 nm. The output mirror is used to allow the second harmonic waves to pass through as the laser output. However, both the output mirror and the input mirror are configured to reflect the wavelengths of the fundamental wave back and forth through the laser resonator, thereby continuing the laser process of the fundamental wavelength.
Although frequency doubling green lasers are commercially available, it has proved to be very difficult to provide a reliable, low noise green laser (see reference 4). In a first example of one of the factors which contributes to the noise problem, the lasant materials used in these lasers typically create a fundamental wave made up of multiple spectral modes (i. e. longitudinal modes which are separated by a multiple of the free spectral range which is determined by the laser cavity geometry). In a homogeneous broadened laser system, these spectral modes will compete for the same gain available in the laser resonator. The gain competition among different longitudinal modes is called "gain cross saturation". In the presence of nonlinear crystals, each longitudinal mode will frequency double itself, for example, 1064.1 nm can be frequency doubled to 532.05 nm. In addition to frequency doubling, two different longitudinal modes can also generate a radiation at a frequency that is the sum of theirs, for example, 1064.1 nm and 1064.0 nm can generate 532.02 nm radiation. The sum frequency generation creates a nonlinear coupling between different longitudinal modes. The gain cross saturation combined with sum frequency generation causes various modes to couple strongly with one another, leading to significant amplitude fluctuations in the second harmonic waves.
Further contributing to the noise problem, the inventor has discovered an additional, previously undefined fundamental wave mode. Specifically, in the case of using Nd:YVO.sub.4 as the lasant material, an additional mode at a wavelength at and/or about 1084 nm is formed by the lasant material.
Popular approaches to remove the so called "green noise" include a single mode laser (see reference 5), two polarization modes laser (see reference 6) and multiple modes (more than 10) laser (see reference 7). However, the single mode approach usually involves a complex ring laser cavity or spectral narrowing elements such as etalons or birefringent tuners. The two mode approach has only been successful at relatively low output powers (tens of mW) and limited to only isotropical lasant materials such as Nd:YAG. The multiple modes approach generally employs a long and bulky laser cavity. The present invention addresses an alternative method to produce a compact, noise free, frequency doubled solid state laser in a cost effective way. "Green noise" is removed in an uniaxial gain medium using two orthogonally polarized modes of the fundamental waves with a filter arrangement.
In a linear cavity, counter-propagating waves at the same frequency will form a standing wave pattern composed of amplitude nodes and antinodes. The nodes are separated by one half of the laser wavelength in the propagating medium. These wave interference effects will introduce the so called spatial hole burning effect in the laser gain medium. In other words, the standing wave pattern will leave undepleted gain available for other modes to develop.
In a typical diode end-pumped solid state laser system, the lasant material is usually close to the input mirror at the end of the resonator. Since the nodes of the standing wave pattern always start from the input mirror, close by longitudinal modes are suppressed because they are nearly in phase with the oscillating mode in the vicinity of the input mirror. These modes slowly move out of phase as they travel away from the input mirror. In a homogeneous broadened laser system, only one longitudinal mode can develop at low pump intensity. However, when the laser system is many times above threshold, since the neighboring modes are not perfectly in phase, many longitudinal modes can develop.
As illustrated in FIG. 1, in the case of the 1084 nm mode (represented by the line indicated by reference numeral 2) and since the 1084 nm mode wavelength is longer than the wavelengths of the multiple 1064 nm modes (represented by the line indicated by reference numeral 4), the dephasing between the 1084 nm mode and the 1064 nm modes is much faster than that between the various 1064 nm modes. Thus the 1084 nm mode can take the undepleted gain from spatial holes burned by the 1064 nm modes and oscillate even at low pump power. Furthermore, even if the 1084 nm mode is not within the frequency doubling bandwidth of the nonlinear crystal, it may not double itself to 542 nm, but it can create a sum frequency radiation with 1064 nm modes at 537 nm. In addition to the mode coupling between different 1064 nm modes described above, mode coupling existing between 1064 nm and 1084 nm modes further increases the amplitude noise problem. The sum frequency generation between the 1064 nm and 1084 nm modes also creates an undesired oscillation at 537 nm.
The present invention provides a laser that significantly reduces these noise problems caused by the coupling of the various modes of the fundamental wave produced by the lasant material.